I use Air Display to convert my iPad in to an extra monitor when I'm not using it. However, I have other iOS devices lying around, too. Is it possible to use Air Display to turn multiple iOS devices in to extra monitors at the same time?

While air display does not allow 2 devices yet, you can install another app called 'Display Pad' and achieve your goal. It's only $2.99 so it really shouldn't be an obstacle. It doesn't have as many features as Air Display, but it gets the job done and by using both apps you can use 2 devices at once on your mac.

My setup is using my macbook Pro with a 24" monitor plugged into it, between those 2 devices I have an ipad in portrait mode running Air Display stacked on some books to elevate it, right below it I have an ipad mini running DisplayPad in landscape mode. All 4 screens work great, I park skype, reminders and itunes into my tablet screens and it's working flawlessly.